In the extraction of oil from wells it is becoming increasingly beneficial to mount pressure and temperature gauges in the completion tubing so that on-line measurements may be made. These are useful for purposes of controlling the flow of oil and water and gas being produced, and planning remedial actions when the well has undesirable attributes. There is also merit in directly measuring the flow of these fluids by some form of flow sensor—but the current art in oil producing practice requires the intervention of the production tubing by insertion of spinner devices fixed to a logging tool. There is also a particular need to measure flow directly at the perforated sections of the production tubing, or even in the open well where stinger mounted sensors may be positioned. Currently there are no operational systems that achieve these direct measurements of flow at the perforations. With the coming of the so-called intelligent or smart completions, downhole control valves are now being used operationally to control the flow of fluids into and out of laterals for water injection and production wells respectively and it is now a more urgent requirement to measure where the water is going and which laterals are contributing to the fluid flow in order to make effective use of these control valves. The traditional methods of using electronic sensors for pressure and temperature have gained a poor reputation due to the unreliability of electronics at the elevated temperatures (up to 190° C.) to be found in the downhole hostile environment. Pressure and temperature gauges for permanent monitoring downhole oil and gas wells are now benefiting from their implementation using passive fibre optic sensor technology which is better able to withstand the harsh temperature environments than electronics systems. Thus the need for on-line downhole fluid flow meters in the high temperature well environment is best suited by the implementation of the flow sensor in fibre optic technology. Although previous attempts at deploying fibre optic techniques are becoming known the subject of the current invention is concerned with the production of the fluid flow into a measurable mechanical quantity that can be instrumented with a plurality of sensor technologies but in particular amenable to fibre optic sensing with the aforesaid benefits. Thus a reliable fibre optic flow sensor is desired for measurement of the (1) low flow rates which may be experienced at the Total Depth (TD) of well, and (2) high rates which may be experienced at the production tubing carrying the fluid to the surface.